Such a wireless information transmitter/receiver includes wireless information storage media (hereinafter referred to as “IC cards”) and a reader/writer device. For example, the wireless information transmitter/receiver can be applied to an automatic ticket gate to check passenger tickets or an entrance control system.
The IC card has a loop antenna, an IC chip connected to terminals of the loop antenna, and a tuning capacitor on a main surface of a plastic card substrate. The reader/writer device includes a loop antenna, a modulator-demodulator circuit, and a CPU. The reader/writer device transmits data to the IC card by flowing electric currents modulated from 13.56 MHz carrier with transmitting data to the loop antenna, and receives data from the IC card by flowing unmodulated electric currents to the loop antenna.
When the loop antenna of the IC card gets closer to the loop antenna of the reader/writer device, the terminal of the loop antenna of the IC card produces induced voltage caused by the electromagnetic induction. Accordingly, the IC chip receives stable power supply voltage and demodulates modulated waves to receive data transmitted from the reader/writer device. When the IC chip transmits data stored in a memory thereof, the IC chip switches terminal impedance by changing the load therein in accordance with the data. The reader/writer device detects the load changes and demodulates them to receive the data from the IC card. Thus, the IC card and the reader/writer device communicate with each other. Subsequently, a higher level system of the reader/writer device, such as a computer or another external information processing system, utilizes the data to, for example, check passenger tickets or control human entrance (refer to Japanese Unexamined Patent Application Publication No. 10-13312).
In a loop antenna 202 of such a reader/writer device, a flow of electric current i produces magnetic fluxes H shown in FIG. 11. The arrow with dashed line shown in FIG. 12 illustrates combined magnetic fluxes of the fluxes around the loop antenna taken along line XII—XII in FIG. 11. That is, inside the loop antenna 202, since directions of the magnetic fluxes generated along each edge of the loop antenna 202 are the same, an upward magnetic field having its maximum value at the center of the loop antenna 202 is generated. In contrast, outside the loop antenna 202, the direction of magnetic fluxes generated along two neighboring edges of the loop antenna 202 is reversed from the direction of magnetic fluxes generated along the opposing neighboring edges. Accordingly, a downward magnetic field magnetic flux is generated after the magnetic fluxes are combined.
Since a power supply of the reader/writer device is an AC power supply of 13.56 MHz, only one side of the loop antenna 202 will be described. A magnetic field shown by a dashed line in FIG. 13 is generated to form communication spaces to communicate with the IC card. That is, as shown in FIG. 13, a main communication space M, also referred to as “main lobe”, having its maximum value at the center of the loop antenna 202, is produced inside the loop antenna 202, while smaller sub communication spaces S, also referred to as “side lobes”, are produced at the periphery of the loop antenna 202.
If, as shown by the arrow in FIG. 13, an IC card 10 passes from the sub lobe S through the main lobe M in one direction (from left to right in FIG. 13), a communication error sometimes occurs.
That is, while the IC card 10 is passing through the first side lobe S, an induced voltage is generated in the IC card 10. The IC card 10 receives initial data from the reader/writer device, and then transmits initial data to the reader/writer device. After the reader/writer device receives the initial data, the IC card 10 passes through a boundary B between the side lobe S and the main lobe M, where a magnetic field sufficient for generating a predetermined induced voltage is not produced. Since power supply for the IC card is temporarily blocked, only the IC card is initialized.
Subsequently, the IC card 10 enters the main lobe M. Although the IC card attempts to receive the initial data from the reader/writer device, the reader/writer device attempts to continue the previous communication in the side lobe S. As a result, the reader/writer device regards this communication mismatch between the IC card and the reader/writer device as a communication error.